In recent years, materials with a hollow structure attract lots of attention due to their wide application prospects in the areas of catalysis, drug delivery and controlled release, the optical field and electrochemical field. The single crystal zeolite with a hollow structure, as a special material, has gained popularity because of its unique porous structure, relatively large specific surface area and porosity, excellent catalytic activity, stability and shape selectivity. In such a hollow single crystal zeolite, the time an object molecule stays in the microporous passages has been greatly shortened, and accordingly the mass transfer performance is improved. At the same time, when compared with a large grain molecular sieve, the small grain molecular sieve has a greater external surface area, shorter porous passage, rich surface acid sites, as well as greatly improved stability, activity and selectivity of catalyst. Therefore, the small grain molecular sieve exhibits relatively better catalytic properties in most important catalytic reactions.
Until now, only a limited number of single crystal ZSM-5 molecular sieves with hollow structure have been reported. Most of them have been prepared with a two-step synthesis approach, in which the ZSM-5 molecular sieve grain that can meet certain requirements is first prepared through a conventional molecular sieve preparation process, and then the obtained zeolite grain is further treated with a liquid of acid or base to obtain the single crystal zeolite molecular sieve with hollow structure. For examples, Mei et al. treated the synthesized zeolite molecular sieve in a size of about 350 nm with a weak base of 0.6 M Na2CO3 to obtain hollow small grain ZSM-5 molecular sieve (Mei C, Liu Z, Wen P, et al. Regular HZSM-5 microboxes prepared via a mild alkaline treatment, Journal of Materials Chemistry, 2008, 18 (29): 3496-3500); Wang et al. first synthesized the nano-level Silicalite-1 zeolite, and subsequently the nano-level Silicalite-1 zeolite was loaded into a ZSM-5 synthesis solution with tetrapropyl ammonium hydroxide as the templating agent for recrystallization, so as to obtain the ZSM-5 molecular sieve with hollow structure (Yongrui Wang, Alain Tuel, Nanoporous zeolite single crystals: ZSM-5 nanoboxes with uniform intracrystalline hollow structures, Microporous and Mesoporous Materials, 2008, 113: 286-295); Song et al. first synthesized the small grain titanium silicalite molecular sieve (TS-1), and used a base of organic quaternary ammonium to treat TS-1, and finally obtained the small grain TS-1 molecular sieve with hollow structure (Song, W., Dai, C., He, Y. , et al. Modification of small-crystal titanium silicalite-1 with organic bases: Recrystallization and catalytic properties in the hydroxylation of phenol. Applied Catalysis A-General, 2012, 453: 272-279); Fodor et al. first synthesiied a nano-scale ZSM-5 molecular sieve, which was then etched with a 0.1 M sodium hydroxide solution and then washed with a 0.1 M hydrochloric acid solution to obtain a hollow nanoscale single crystal ZSM-5 zeolite (Fodor D, Pacosova L, Krumeich F, et al. Facile synthesis of nano-sized hollow single crystal zeolites under mild conditions, Chemical Communications, 2014, 50 (1): 76-78); the patent CN102491366A reported the synthesized monodisperse ZSM-5 nanoscale zeolites with various ratios of silicon to aluminum and various particle sizes were mixed with the aqueous alkaline solutions with different concentrations, which were then stirred at various temperatures for a certain period of time, so as to form a regular cavity structure, and the cavity size of the zeolite can be altered by way of adjusting the treatment conditions.
In light of the foregoing, it can be seen that although there are certain reports on the zeolite molecular sieves with hollow structure, their synthesis and preparation processes are relatively complex. In addition, it is necessary to add certain solutions of acid or base for treatment in order to obtain the respectively molecular sieves with certain hollow structure. These preparation processes waste energy, consume time and pollute the environment.